AbstractProcessStep: Make init fail if a process is still running

This should not happen, but better save than sorry.

Task-number: QTCREATORBUG-17984
Change-Id: I3efa8fb38a915ae29366f3ab715077b762d53828
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/projectexplorer/abstractprocessstep.cpp

@@ -158,7 +158,7 @@ void AbstractProcessStep::setIgnoreReturnValue(bool b)
 bool AbstractProcessStep::init(QList<const BuildStep *> &earlierSteps)
 {
     Q_UNUSED(earlierSteps);
-    return true;
+    return !m_process;
 }
 
 /*!